1. Field of the Invention
This invention relates to a ring spring suitable for use in a safety device for an automobile power window, and a power transmission device using the ring spring.
2. Description of the Prior Art
A safety device has conventionally been incorporated in a power window for automotive vehicles to interrupt an ascending operation of a window glass when foreign matter is caught between the window glass and a window frame. FIG. 11 schematically illustrates one of such safety devices. A worm wheel 90 is connected to the side of a drive mechanism for ascending and descending the window glass. A drive shaft 94 is connected to the operation side, namely, a regulator (not shown) for the window glass. A rubber damper 91 and a metal plate 92 are disposed in the worm wheel 90. The drive shaft 94 extends through central holes of the worm wheel 90 and damper 91 to be loosely fitted in them, whereas it is tightly fitted in a central hole of the plate 92 for rotation therewith.
The worm wheel 90 is rotated when a drive device is turned on so that the power window is closed. The damper 91 in engagement with the worm wheel 90 is simultaneously rotated. Since engagement pieces 97 of the plate 92 are engaged with grooves 98 of the damper 91 respectively, the plate 92 is also rotated with the damper 91. The drive shaft 94 is thus rotated so that the window glass is ascended by the window regulator (not shown).
A drive motor is subjected to a large braking force when foreign matter is caught between the window glass and an upper edge of the window frame during ascent of the window glass. In this case, gear teeth of the worm wheel 90 would be chipped without the damper 91. As shown in FIG. 11, however, the damper 91 is interposed between the worm wheel 90 and the plate 92. When a rotor of the drive motor is locked, the damper 91 is deformed to be spread such that a phase shift occurs between the damper 91 and the plate 92 in the stopped state. In other words, part of the rotational energy of the worm wheel 90 is converted to energy deforming the damper 91 so that a damping force is obtained. Thus, the gear teeth of the worm wheel 90 can be prevented from being chipped. Suitable detecting means are provided for detecting a reduction in the rotational speed of the drive motor in mesh engagement with the worm wheel 90 due to the above-described energy conversion, thereby generating a signal. The signal is supplied to a control device, which delivers an output signal to interrupt or reverse-rotate the drive motor. Consequently, foreign matter can be prevented from being caught between the window glass and the window frame.
The performance of the drive motor is set so that the window glass can be ascended and descended even when the window glass is frozen. Accordingly, the damper 91 needs to provide a damping force necessary for absorbing shock due to the locked-rotor condition, for example, when the foreign matter has been caught between the window glass and the window frame or when the window glass has completely been closed up. To ensure the necessary damping force, the prior art has proposed to render the damper large in size. However, this cannot be adopted because of strong demand for space saving.
Furthermore, the damper needs to be fit for repeated use. Additionally it is desired that, the damper has a linear damping characteristic since the detection of abnormal conditions are based on differentials.
However, the conventional damper is made of rubber as described above. Accordingly, since the rubber damper is deteriorated or hardened due to long-time use, the damping force of the damper 91 is decreased relative to the rotational force of the worm wheel 90. Consequently, an expected performance cannot be achieved from the damper. This necessitates an early replacement of the damper.
Furthermore, it is difficult to obtain a large reaction force from a small rubber damper. Additionally, the rubber damper has a non-linear damping characteristic. Thus, the conventional rubber damper is not always suitable in view of the above-described usage thereof.
A damper comprising a metal spring has been proposed in order that the above-described problems may be solved. More specifically, the metal spring is generally less deteriorative than the rubber damper and has a linear damping characteristic. Thus, the above-described problems can be solved by the use of a metal damper.
The spring is bent into a ring spring in view of the shape and location of disposition of the safety device when incorporated in the safety device. In this case, the conventional safety device can be used without substantial alterations in design. Furthermore, the ring spring should be closed in its initial state with both free ends thereof being abutted against each other so that variations in the dimensions of products can be restrained by an effective use of the torque acting on the drive shaft.
However, when a single strip spring is bent into the shape of a closed ring, both ends thereof are opened by spring back. Consequently, the torque of the ring spring cannot efficiently be utilized.
However, both ends of a single strip spring may be bent into a ring spring in view of the spring back, as shown in FIGS. 9 and 10. FIG. 9 illustrates a ring spring 70 formed by bending a single strip spring. The ring spring 70 has notches 86 and 87 formed in both ends thereof respectively. Each of the notches 86 and 87 has a width half that of the ring spring 70. The ring spring 70 has tongues 73 and 74 formed on both ends thereof respectively. The distal ends of the tongues 73 and 74 serve as free ends and are bent into a U-shape so that abutment strips 71 and 72 are provided. The tongues 73 and 74 are formed to be located high and low respectively and complementary to each other in view of the spring back, as viewed in FIG. 9, whereupon the ring spring 70 can be designed so that the abutment strips 71 and 72 are abutted against each other.
FIG. 10 illustrates another ring spring 80 formed by bending a single strip spring. The ring spring 80 has at one end thereof one small strip 84 and two notches 88 which are formed in both sides of the ring spring and each of which has a width one third that of the ring spring. The ring spring 80 further has at the other end thereof two small strips 83 and a central notch 89 which has a width one third that of the ring spring. Ends of the three strips 83 and 84 serve as free ends and are bent into a U-shape so that abutment strips 81 and 82 are provided. Thus, the ring spring 80 can be formed by bending the strip spring so that the strips 83 and 84 and the notches 88 and 89 are complementary.
However, the torque of the ring spring 70 and 80 cannot efficiently be utilized, because each of the two ring springs 70 and 80 has notches 86, 87 and 88 in both ends